1. Field of the Invention
The present invention relates to a process for the selective ring opening of aromatic compounds using a mesoporous catalyst system.
2. Background of the Art
Aromatic saturation and hydrocracking have been proven to be upgrading technologies for improvement of diesel fuel cetane quality. Unfortunately, aromatics saturation brings about a marginal improvement in cetane number and reduction of the density of distillate fuels. By hydrocracking naphthalenes and their alkyl homologues into the jet and naphtha boiling ranges, one achieves a net increase in high cetane value distillate components (e.g. alkyl cyclohexanes, alkyl benzenes, paraffins, and slightly branched paraffins). The primary debit for aromatics saturation is its limited cetane improvement and high hydrogen consumption per cetane barrel improvement. The primary debit for hydrocracking is its poor selectivity for retaining distillate and total liquid products at the expense of C3/C4 production.
It has been widely reported [e.g., McVicker et al., J. Catal., 210, 137 (2002)] that the anticipated U.S. environmental regulations will require diesel specification of specific gravities <0.85 and cetane numbers>45, and European diesel fuels will require cetane numbers of 55 or more. Aromatics saturation does improve the cetane number to some extent. However, selective ring opening (“SRO”) of naphthenic molecules to alkylcyclohexanes, n-paraffins and slightly branched paraffins significantly improves the cetane number of the diesel fuel. In the SRO process, naphthenic rings are ideally opened to alkylcyclohexanes as well as straight and branched alkanes with only minor loss of molecular weight.
U.S. Pat. No. 5,763,731 to McVicker et al. is directed to a process for selectively opening naphthenic rings. A process is disclosed for selectively opening rings of ring compounds in a feed stream wherein at least about 50 wt % of the ring compounds in the feed stream are characterized as containing at least one C6 ring having at least one substituent containing 3 or more carbon atoms, which substituents are selected from the group consisting of fused 5-membered rings; fused 6-membered rings; C3 or greater alkyls, cycloalkyls; and aryl groups. This patent also claims a bifunctional catalyst system for this process, which is comprised of an effective amount of a metal selected from Ir, Ru, Rh or mixtures thereof, on a catalyst support and wherein the catalyst support contains an acidic function selected from the group of silica, silica-alumina or zeolite having a structure characteristic of faujasite structure with a high Si/M ratio (M is Al, Ga, B, Zn, Fe or Cr) above 30. The acidic function can be incorporated into the catalyst or be a separate catalyst. However, for such a high Si/M ratio, the faujasite must be post-treated after synthesis to remove most of the framework M component. McVicker et al. also teach that a controlled amount of acidity is used to isomerize the cyclo-C6 components to cyclo-C5 components, which then can be ring opened more easily. The control of acidity is an important factor in producing a selective ring opening catalyst as excessive acidity leads to cracking instead of hydrogenolysis (carbon-carbon bond cleavage).
U.S. Pat. No. 5,811,624 to Hantzer et al. discloses a process of selectively opening five- and six-membered rings without substantial cracking using a transition metal such as Mo and W supported on a carbide, nitride, oxycarbide, oxynitride or oxycarbonitride and a noble metal supported on the same support or a separate carrier. Hantzer et al. claim to have better selectivity towards ring opening without a decrease in carbon number, compared to the noble metal based systems as, for example, claimed in McVicker's patents.
U.S. Pat. No. 6,241,876 to Tsao et al. describes a process for selective ring opening wherein the catalyst consists of a large pore molecular sieve having a faujasite structure and an alpha acidity of less than one, preferably less than 0.3, and the noble metal is selected from group VIII of the periodic table. The very low acidity of their catalyst is regarded as an essential step to minimize ring opening yield losses due to cracking.
Furthermore, U.S. Pat. No. 6,623,626 to Baird et al. discloses a process for ring opening using a combination of two catalysts, wherein the first one is an isomerization catalyst with an oxide supported naphthene ring isomerization metal and the second one is a ring opening catalyst comprising iridium supported on an inorganic oxide. The two catalysts are stacked or physically mixed together. The authors claim an improved ring opening yield of the iridium based ring opening catalyst, when the C6 rings are first isomerized to a C5 ring by the isomerization catalyst. In contrast to U.S. Pat. No. 5,763,731, they describe an improved quality of the obtained ring opened product, as the fraction of linear, unbranched alkanes is increased.
So far, the prior art has always described either the use of zeolitic supports for the ring opening of naphthenic molecules or the use of bulk oxides like silica or alumina. The same is true for the isomerization of cyclohexane components to methylcyclopentane components. Therefore, the support materials had either a restricted access for large molecules (e.g., zeolitic support), resulting in diffusion limitations or had a lower surface area, as it is typical for the bulk oxides.